Reduction of styrene emissions remains a key issue in open mold processes using styrene-containing materials such as unsaturated polyesters, vinyl esters and other thermosetting resins. One of the largest areas of applications is the open mold process, particularly hand lay-up, spray-up, non-reinforced castings, gelcoats, filament winding and the like. New environmental concerns; however, demand better control on the emissions of organic compounds into the environment. This is prompting the polymer industry to find ways to develop technologies that can provide less potential hazards to workers in contact with the thermosetting resins. At the same time, the market requires that the new products should have minimal increase in cost when commercialized and do not compromise reactivity of the resins. Important is that all materials should also have good compatibility with all components in the mixtures. Viscosities should stay within an acceptable range so that pouring or spraying is not compromised. Wetting of glass or fillers also need to be maintained and physical properties should be similar or better than the standard materials currently being used.
Several methods have been proposed as possible ways to reduce styrene to minimize monomer emissions during the curing process of unsaturated polyesters or vinyl esters. One common method is the replacement of styrene by another reactive diluent that can produce fewer emissions during curing. This approach can lead to systems with slower reactivity, incomplete curing and higher costs. Reducing the amount of styrene or reactive diluent has been used as an attempt to reduce emissions. However, this approach leads to higher viscosities, making more difficult for hand-lay-up, rolling or spraying of the resins.
Another approach involves the preparation of low molecular weight polymers. Polymers with lower molecular weight are more soluble in styrene or other reactive diluent yielding lower viscosities and therefore requiring lower amount of diluents. Problems associated with lower molecular weight thermosetting systems are that the resulting physical properties of the final products are often compromised. Overall, products typically have inferior performance comparing to those of higher molecular weight components.
Another common approach also used in the reduction of styrene emissions is adding waxes to the thermosetting resins. Waxes limit the elimination of diluent vapors during the curing, however, one of the problems encountered with this approach is that there is poor interlaminate bonding.
The esterification of hydroxyl containing polycondensates with unsaturated carboxylic acids have become increasingly interesting commercially as coating resins and other coating materials, owing to their being solvent free and having easy processability. These esters are (meth)acrylic acid esters which are based on polyhydric alcohols and oligoesters formed from polyhydric alcohols reacted with polyfunctional acids or anhydrides. Their area of application is mainly in coating compositions curable by UV or electron beam. One preferred method of preparing (meth)acrylates is the direct esterification of the polyhydric aligo-alcohols with acrylic or methacrylic acid in the presence of esterification catalyst and of a solvent which forms and azeotropic mixture with water entraining agent. Typical reaction temperatures can range from 90° C. to 150° C. High reaction temperatures require a large amount of inhibitors in order to achieve good yields of the esterification products and for effectively suppressing the polymerization of (meth)acrylic acid esters. In addition to adding inhibitor, a stream of air needs to be added to maintain the inhibitors active and prevent polymerization of the (meth)acrylate intermediates. The polymerization inhibitors in combination with air and high temperatures generate a strong color in the reaction mixture, therefore making difficult to prepare materials with low color. The dark end products need to be washed with color scavenging compounds, which is time consuming, reduces the ester yield, increases the requirement of solvents as water containing agent, which have to be distilled again, and moreover leads to a high level of pollution and waste water.
Exemplary prior art references include U.S. Pat. Nos. 5,874,503 and 4,546,142 and describe the use of waxes with a variety of unsaturated polyester resins. The wax is pre-dispersed in the resin and during the curing process, the wax forms a thin film on the laminates prepared. The film of wax act as a barrier preventing styrene from evaporating at the moment of curing the laminates. A disadvantage on using waxes is that the wax separates from the resin when the resin mixture is exposed to cold temperatures, becoming inefficient at the time of curing the composite systems.
U.S. Pat. Nos. 5,393,830, 5,492,668, and 5,501,830 propose laminating resins which employs a reduce amount of styrene so as to meet a specified volatile emission level according to test standards. The disclosed resin mixtures include a polyester resin, ethylene glycol dimethacrylate, vinyl toluene, cyclohexyl methacrylate, and bisphenol dimethacrylate. The compositions require high cost diluents and have more difficulty in wetting fibers.
U.S. Pat. No. 6,468,662 describes using a low molecular weight epoxy acrylate in combination with reduce amount of styrene and methacrylate monomers. Glass fiber wetting is improved but cost may be compromised in certain applications.
U.S. Pat. Nos. 5,118,783 and 6,107,446 and U.S. Patent Publication No. 2004/068088, describe the preparation of unsaturated polyesters with low molecular weight. As stated above, resins with low molecular weight and low styrene content may compromise physical properties of the resulting cured materials.
Other approaches to control the molecular weight and add reactivity to the molecules are by end-capping the polymers with unsaturated monomers. U.S. Pat. Nos. 5,096,938 and 6,150,458 describe end-capping of polyester polyols with (meth)acrylic acid or their alkyl esters. A different approach is proposed in U.S. Pat. Nos. 5,373,058 and 5,747,607, where glycidyl methacrylate is used to react with polyesters containing acid end groups.
U.S. Patent Publication Nos. 2004/00776830 and 2007/0179250 propose the preparation of low molecular weight saturated polyester polyols end-capped with at least one (meth)acrylic acid. The esterification process requires a large amount of inhibitors and air during the process which leads to dark products. To obtain good physical properties, the (meth)acrylate intermediates are mixed with styrene.
U.S. Pat. No. 6,153,788 describes the preparation of monohydric and polyhydric alcohols and polyesters reacted with (metha)acrylate end groups. The esterification is carried in the presence of an esterification catalyst, phenolic inhibitors, a solvent to help azeotropically remove the water generated, air is passed though the reaction medium, and a monofunctional epoxy to neutralize the mixture. High viscosities are reported for the polyester acrylate intermediate is reported.
U.S. Pat. No. 6,458,991 proposes the preparation of hydroxyl containing polyfunctional intermediates esterified with acrylic or methacrylic acid in the presence of esterification catalyst, hypophosphorus acid, a Cupper salt, a solvent and an air flow passed through the mixture. Acid neutralization is done using calcium oxide and sodium sulfide. Problems are encountered with the process removing completely the calcium oxide and having residual sodium sulfide which prevents crosslinking of the acrylate intermediate under room temperature curing conditions.
U.S. Pat. Nos. 6,063,957, 6,150,458, and 5,821,383 describe hydroxyl containing polyfunctional alcohols and polyesters, esterified acrylic acid, an esterification catalyst, a solvent, peholic inhibitors, antioxidants and in the presence of air. Amines are used to scavenge the catalyst and residual acid. It is common to observe that amines in the presence of air and temperature oxidize thereby increasing the color of the mixtures.
U.S. Pat. No. 6,268,467 describes unsaturated polyester resins for gelcoat applications having a number average molecular weight of 700-2500 and a weight average of 2600 to 6000. The resins are dissolved in a styrene content ranging from 28 to 35%. The low styrene content is added to minimize the problem with volatile organic emissions (VOC).
U.S. Patent Publication No. 2009/0022998 describes unsaturated polyester for gelcoat applications containing styrene in a concentration as low as 28%. The low styrene content is added to control the VOC emissions.
U.S. Patent Publication No. 2009/076218 describes unsaturated polyesters for gelcoat applications end-capped with glycidyl methacrylate having a number average molecular weight from about 500 to 2500. The references propose that if the molecular weight is less than 560, the curing and gelcoat properties will be poor. In addition, if the molecular weight is higher than about 2500, the resulting resin will have a high viscosity and can not be used to make a low VOC gelcoat. The reference proposes styrene content of up to 30% in combination with 30 percent 1,6-hexanediol diacrylate.
There; however, remains a need in the art to address the various problems noted above in producing vinyl-containing compounds. Specifically, it would be advantageous to minimize the color of the resulting materials, obtain products with high reactivity that can undergo crosslinking at room temperature and also provide a process that does not require the extra steps often required in forming vinyl-containing compounds such as, for example, extraction, separation, filtration and/or washing. Such a process could advantageously be employed in the preparation of gelcoats, and applied in a number of other applications such as, for example, sheet molding compounding (SMC) resins, castings resins, UV cured resins and adhesives, pultrusion resins, corrosion resistant resins, flame retardant resins, low or zero styrene content resins, filament winding, hand lay-up, resin transfer molding, prepregs, coating resins and the like.